This invention relates to geophysical prospecting using seismic signals, and in particular to systems and methods for estimating seismic velocity.
Effectively searching for oil and gas reservoirs often requires imaging the reservoirs using three-dimensional (3-D) seismic data. Seismic data is recorded at the earth""s surface or in wells, and an accurate model of the underlying geologic structure is constructed by processing the data. 3-D seismic imaging is perhaps the most computationally intensive task facing the oil and gas industry today. The size of typical 3-D seismic surveys can be in the range of hundreds of gigabytes to tens of terabytes of data. Processing such large amounts of data often poses serious computational challenges.
Obtaining high-quality earth images necessary for contemporary reservoir development and exploration is particularly difficult in areas with complex geologic structures. In such regions, conventional seismic technology may either incorrectly reconstruct the position of geological features or create no usable image at all. Moreover, as old oil fields are depleted, the search for hydrocarbons has moved to smaller reservoirs and increasingly hostile environments, where drilling is more expensive. Advanced imaging techniques capable of providing improved knowledge of the subsurface detail in areas with complex geologic structures are becoming increasingly important.
Obtaining high-quality images of subsurface structures typically requires having an accurate velocity model of the subsurface region of interest. One commonly-used method of improving the accuracy of velocity models is Migration Velocity Analysis (MVA). A known MVA approach is based on Common Reflection Point (CRP) gathers generated by 3-D prestack Kirchhoff migration. For further information on CRP gathers see Stork, xe2x80x9cReflection Tomography in the Postmigrated Domain,xe2x80x9d Geophysics 57:680-692 (1992), and Deregowski, xe2x80x9cCommon Offset Migration and Velocity Analysis,xe2x80x9d First Break 8(6):224-234 (1990). The CRP gathers contain redundant structural information which can be used to correct the velocity model. In a conventional approach, a residual velocity value that corresponds to a horizontal alignment of events on a CRP is computed for a plurality of locations within the velocity model. The computer residual velocity values are then employed in updating the velocity model. Conventional MVA using CRP gathers can suffer from accuracy and complexity problems, however.
The present invention provides a geophysical velocity analysis method comprising the steps of: establishing a seismic data set and a velocity model corresponding to a seismic exploration volume; generating a set of common image gathers for the volume from the seismic data set and the velocity model; computing a velocity parameter value for each of the common image gathers; tying the velocity parameter value to a geological horizon; and updating the velocity model using the velocity parameter value tied to the geological horizon.
Tying the velocity parameter value to the geological horizon can include snapping the velocity parameter to the geological horizon, or vertically interpolating the velocity parameter value along the geological horizon from velocity parameter values for neighboring points corresponding to common image gathers. The velocity parameter is preferably a residual velocity or slowness.